1. Technical Field
The present invention relates to an oscillator circuit, a vibratory device, an electronic apparatus, a moving object, a method of adjusting a vibratory device, and a sensitivity adjustment circuit.
2. Related Art
Voltage controlled oscillators (VCO) capable of varying the oscillation frequency in accordance with a control voltage are widely known, and are used for a variety of purposes. Among these oscillators, a voltage controlled X'tal oscillator (VCXO) using a crystal vibrator is high in frequency stability, and is used for a variety of purposes.
In general, in the voltage controlled oscillator (VCO), since the resolution of the frequency adjustment is deteriorated if the frequency control voltage sensitivity (Vc sensitivity) defined as the variation of frequency to the variation of the frequency control voltage (Vc) is too high, and the range in which the frequency can be adjusted becomes insufficient if the Vc sensitivity is too low, the Vc sensitivity is required to be at a desired level. However, in reality, since the Vc sensitivity is not the same between individual voltage controlled oscillators due to manufacturing factors and so on, adjustment of the Vc sensitivity is necessary for each of the voltage controlled oscillators.
Further, the adjustment of the Vc sensitivity is also necessary in the case of making it possible to deal with a plurality of types of vibrators different in vibrator sensitivity. The vibrator sensitivity SXtal, the frequency control voltage sensitivity (the Vc sensitivity) SV, and the adjustment frequency ΔF are calculated by Formulas (1) through (3) below, respectively.
                              S          Xtal                =                                            -                              C                1                                                    2              ·                                                (                                                            C                      0                                        +                                          C                      L                                                        )                                2                                              ×          1          ⁢                                          ⁢                                    E              6                        ⁡                          (                              ppm                /                pF                            )                                                          (        1        )                                          S          V                =                              S            Xtal                    ×                                    Δ              ⁢                                                          ⁢                              C                L                                                    Δ              ⁢                                                          ⁢                              V                C                                              ⁢                      (                          ppm              /              V                        )                                              (        2        )                                          Δ          ⁢                                          ⁢          F                =                              ∫            Vc                          Vc              +                              Δ                ⁢                                                                  ⁢                Vc                                              ⁢                                    S              V                        ×                                                  ⁢                          ⅆ                              Vc                ⁡                                  (                  ppm                  )                                                                                        (        3        )            
In Formulas (1) through (3), C0 denotes a vibrator parallel capacitance, C1 denotes a vibrator series capacitance, CL denotes an oscillator load capacitance, and VC denotes the frequency control voltage.
According to Formulas (1) through (3), if attempting to deal with a plurality of types of vibrators different in vibrator sensitivity, it is necessary to perform a gain adjustment of the frequency control voltage Vc, an adjustment of CL, or an adjustment of ΔCL/ΔVc.
In the past, as described in, for example, JP-A-9-102714 (Document 1), it is commonly performed that the value of CL is changed by changing the value of the load capacitance connected to each of the input side and the output side of the vibrator to thereby adjust the Vc sensitivity.
FIGS. 19A and 19B are circuit diagrams of the related art oscillators using an inverter and a bipolar transistor as an amplifier element, respectively. In either of the cases, a variable capacitance element (a varactor) having a capacitance value varying in accordance with the frequency control voltage Vc and a capacitance bank composed of a plurality of capacitance element and a plurality of switches are connected to the both ends (the both ends of the amplifier element) of the vibrator, and by adjusting the capacitance value of the capacitance bank in accordance with a setting value stored in a memory, the Vc sensitivity is adjusted so as to have a desired level.
The variation of the capacitance value to the variation of the electrical potential difference between the both ends of the variable capacitance element has the maximum value when the electrical potential difference between the both ends is a certain electrical potential difference Vt. Therefore, the Vc sensitivity due to the variable capacitance element on the input side of the amplifier element takes the maximum value when the control voltage Vc is shifted by Vt from electrical potential V1 on the input side of the amplifier element. On the other hand, the Vc sensitivity due to the variable capacitance element on the output side of the amplifier element takes the maximum value when the control voltage Vc is shifted by Vt from electrical potential V2 on the output side of the amplifier element. The overall Vc sensitivity of the oscillator is obtained by combining the two Vc sensitivities, and becomes the characteristic corresponding to the electrical potential difference between V1 and V2. It should be noted that although Vt varies depending on the characteristics of the variable capacitance elements, the explanation will hereinafter be presented assuming Vt=0 in order to simplify the explanation.
FIG. 20A is a diagram showing an example of a relationship between the frequency control voltage Vc and the adjustment frequency before and after the Vc sensitivity adjustment in the case of using a vibrator high in vibrator sensitivity, and FIG. 20B is a diagram showing an example of a relationship between the frequency control voltage Vc and the Vc sensitivity corresponding to FIG. 20A.
In the example shown in FIGS. 20A and 20B, the Vc sensitivity is adjusted by increasing the load capacitance so that the frequency varies in a range of −15 ppm through +15 ppm with respect to the nominal frequency when varying the frequency control voltage Vc in a range of ΔV centered on 0.9V. As shown in FIG. 20B, although the Vc sensitivities due to the variable capacitance elements on the input side and the output side of the amplifier element take peak values when the frequency control voltage Vc takes V1 and V2, respectively, the peak values after the adjustment are lowered compared to the peak values before the adjustment. Thus, it has been achieved to set the overall Vc sensitivity of the oscillator to a value around the target value of 50 ppm/V with respect to the variation range ΔV of the frequency control voltage Vc. As described above, according to the related art adjustment method, in the case of using the vibrator high in vibrator sensitivity, by increasing the load capacitance value by a value corresponding to the increment in the vibrator sensitivity, the Vc sensitivity can be adjusted to the target value.
Incidentally, the phase noise in an output signal of the voltage controlled oscillator (VCO) is calculated by an SSB phase noise reduction formula provided by Formula (4) below.
                              L          ⁡                      (                          f              1                        )                          =                  10          ⁢                                          ⁢          log          ⁢                                    1              2                        ⁡                          [                                                {                                                                                    (                                                                              f                            0                                                                                2                            ⁢                                                                                                                  ⁢                                                                                          Q                                L                                                            ·                                                              f                                1                                                                                                                                    )                                            2                                        +                    1                                    }                                ×                                  (                                                            f                      a                                                                                      f                        1                                            +                      1                                                        )                                ×                                                      4                    ⁢                                                                                  ⁢                    FKT                                                        P                    0                                                              ]                                ×          10          ⁢                                          ⁢          log          ⁢                                    1              2                        ·                                                                                (                                          V                      cnoise                                        )                                    2                                ⁢                                                      (                                          S                      V                                        )                                    2                                                            f                1                2                                                                        (        4        )            
In Formula (4), f0 denotes a resonant frequency, f1 denotes a mistuned frequency, QL denotes a loaded Q, fα denotes a corner frequency, F denotes a noise figure, K denotes the Boltzmann constant, T denotes ambient temperature, P0 denotes an oscillation power, and Vcnoise denotes the Vc noise.
According to Formula (4), assuming that the Vc noise (Vcnoise) is constant, the lower the Vc sensitivity (SV) is, the smaller the phase noise becomes. Therefore, there recently exists a purpose of using the oscillator adjusted to have a low Vc sensitivity in order to reduce the phase noise, and in some cases, adjustment to the low Vc sensitivity is required.
FIG. 21A is a diagram showing an example of the relationship between the frequency control voltage Vc and the adjustment frequency before and after the Vc sensitivity adjustment in the case of using the oscillator in the low Vc sensitivity, and FIG. 21B is a diagram showing an example of the relationship between the frequency control voltage Vc and the Vc sensitivity corresponding to FIG. 21A.
In the example shown in FIGS. 21A and 21B, before the adjustment, the Vc sensitivity is 50 ppm/V, and when varying the frequency control voltage Vc in a range of ΔVA centered on 0.9V, the frequency varies in a range of −15 ppm through +15 ppm with respect to the nominal frequency. When adjusting the Vc sensitivity so as to drop to the target value of 40 ppm by increasing the load capacitance, after the adjustment, when varying the frequency control voltage Vc in a range of ΔVB centered on 0.9V, the frequency varies in a range of −15 ppm through +15 ppm with respect to the nominal frequency. In other words, in the case of decreasing the Vc sensitivity, it is required to increase the variation range of the frequency control voltage Vc in order to keep the frequency adjustment range constant. As shown in FIG. 21B, although the Vc sensitivity can be kept at a value around the target value of 40 ppm/V in the vicinity of 0.9V, which is the center of the variation range ΔVB of the frequency control voltage Vc, the drop of the Vc sensitivity on the low voltage side and the high voltage side is increased. As described above, according to the adjustment method of the related art, in the case of using the oscillator in the low Vc sensitivity, even if the Vc sensitivity is lowered by increasing the load capacitance value, it results that the Vc sensitivity is not constant in a desired range of the frequency control voltage Vc, but fluctuates. The variation of the Vc sensitivity becomes a factor for obstacle for a stable operation of the oscillator and characteristic degradation of C/N and so on.
Further, there are also the case in which the input/output amplitude voltage is different between the oscillators due to, for example, the difference in vibrator characteristics and the oscillation stage current, and the case in which the oscillator is used with the oscillation amplitude reduced in order to realize low power consumption of the oscillator, and it is required that the adjustment of the Vc sensitivity can accurately be performed irrespective of the level of the oscillation amplitude.
As shown in FIGS. 22A and 22B, the level of the oscillation amplitude affects the characteristics of the Vc sensitivity. FIG. 22A is a diagram showing an example of a relationship between an electrical potential difference between the both ends of the variable capacitance element when changing the oscillation amplitude and the capacitance, and FIG. 22B is a diagram showing an example of the relationship between the frequency control voltage Vc and the Vc sensitivity when changing the oscillation amplitude.
As shown in FIG. 22A, the smaller the oscillation amplitude is, the steeper the gradient of the capacitance variation of the variable capacitance element becomes, and as a result, as shown in FIG. 22B, the smaller the oscillation amplitude is, the higher the Vc sensitivity becomes, but the worse the linearity becomes.
FIG. 23A is a diagram showing an example of the relationship between the frequency control voltage Vc and the adjustment frequency before and after the Vc sensitivity adjustment in the case of using the oscillator in the small oscillation amplitude, and FIG. 23B is a diagram showing an example of the relationship between the frequency control voltage Vc and the Vc sensitivity corresponding to FIG. 23A.
In the example shown in FIGS. 23A and 23B, the adjustment is performed by increasing the load capacitance so that the frequency varies in a range of −15 ppm through +15 ppm with respect to the nominal frequency when varying the frequency control voltage in a range of ΔV centered on 0.9V. As shown in FIG. 23B, although it has been achieved that the Vc sensitivity is decreased to a value in the vicinity of the target value of 50 ppm/V with respect to the variation range ΔV of the frequency control voltage Vc, the linearity is not improved. As described above, according to the adjustment method of the related art, in the case in which the oscillation amplitude is small, even if the Vc sensitivity is lowered by increasing the load capacitance value, it results that the Vc sensitivity is not constant in a desired range of the frequency control voltage Vc, but fluctuates. The variation of the Vc sensitivity becomes a factor for obstacle for a stable operation of the oscillator and characteristic degradation of C/N and so on.